Although the present invention is equally applicable to the determination of resistance, inductance or capacitance and may be employed in a wide variety of applications, the following description of the present invention and reference to background of invention is primarily directed to the field of ultraviolet light intensity measurement employing a very high resistance transducer.
In the field of water purification and contamination measurement, it is conventional to employ a system incorporating an ultraviolet light source with an ultraviolet light detector spaced therefrom by a liquid. The function of the detector is to measure or monitor the ultraviolet light intensity illuminating the medium. Sensing of ultraviolet light intensity is accomplished with a transducer and, for commercial applications, it is important to minimize the cost and maximize the reliability of such a transducer while at the same time ensuring that it is only ultraviolet light that is being measured. There has been developed an ultraviolet light sensor which is responsive substantially to ultraviolet light and which is also relatively inexpensive. A difficulty with employing this type of sensor is the very high resistance of the sensor. The resistance of the sensor varies inversely with intensity of incident ultraviolet light and may typically vary in the range of 0.1 megohms to 1,000 megohms.
In general, resistance, as of transducers, sensors, or the like, is measured by applying a voltage across or current through the device and measuring the current flow or voltage drop, with the resistance then being determined as the quotient of the voltage-to-current. In order to achieve reliability in the measuring circuitry, it is desired to employ solid state circuitry; however, in the resistance range of interest herein, a linear amplifier having a discrete input transistor will have the leakage current or offset voltage of the same order of magnitude as the signal level to be determined, so that accurate measurement is not possible. It has been proposed to employ field effect transistors which have an extremely high input impedence and, consequently, low leakage current. However, for linear amplification of dc voltages, filed effect transistors exhibit two basis modes of instability: (a) they are temperature sensitive with respect to bias; and (b) the operational parameters vary considerably from manufacturing lot to lot. Compensation for these instabilities for linear amplification of dc voltages is generally impractical.
It is also noted that various sensors, including the one briefly commented upon above, have a hyperbolic signal response. Consequently, conventional linear measuring systems produce an output which is generally impractical to utilize because of dynamic range and/or sensitivity limitations.
The present invention provides a measuring circuit for monitoring, for example, very small variations over a large dynamic range of resistance without employing linear current amplification and having a circuit configuration intrinsically providing a matched hyperbolic signal response such that utilization of same with a transducer or circuit element (as mentioned above) produces a linear output.